LNA gain adjustment in an RF receiver to compensate for intermodulation interference

ABSTRACT

A Radio Frequency (RF) receiver includes a low noise amplifier (LNA) and a mixer coupled to the output of the LNA. The gain of the LNA is adjusted to maximize signal-to-noise ratio of the mixer and to force the mixer to operate well within its linear region when an intermodulation interference component is present. The RF receiver includes a first received signal strength indicator (RSSI_A) coupled to the output of the mixer that measures the strength of the wideband signal at that point. A second received signal strength indicator (RSSI_B) couples after the BPF and measures the strength of the narrowband signal. The LNA gain is set based upon these signal strengths. By altering the gain of the LNA by one step and measuring the difference between a prior RSSI_B reading and a subsequent RSSI_B′ reading will indicate whether intermodulation interference is present.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of and claims priority to U.S. patentapplication having an application Ser. No. 11/523,332; filed Sep. 19,2006; which application is a continuation of and claims priority to U.S.patent application having an application Ser. No. 09/966,060; filed Sep.28, 2001, now U.S. Pat. No. 7,120,410; and in which both applicationsare incorporated herein by reference in this application.

SPECIFICATION

1. Field of the Invention

This invention relates generally to wireless communications; and moreparticularly to the operation of a Radio Frequency (RF) receiver withina component of a wireless communication system.

2. Background of the Invention

The structure and operation of wireless communication systems isgenerally known. Examples of such wireless communication systems includecellular systems and wireless local area networks, among others.Equipment that is deployed in these communication systems is typicallybuilt to support standardized operations, i.e. operating standards.These operating standards prescribe particular carrier frequencies,modulation types, baud rates, physical layer frame structures, MAC layeroperations, link layer operations, etc. By complying with to theseoperating standards, equipment interoperability is achieved.

In a cellular system, a governmental body licenses a frequency spectrumfor a corresponding geographic area (service area) that is used by alicensed system operator to provide wireless service within the servicearea. Based upon the licensed spectrum and the operating standardsemployed for the service area, the system operator deploys a pluralityof carrier frequencies within the frequency spectrum that support thesubscribers' subscriber units within the service area. These carrierfrequencies are typically equally spaced across the licensed spectrum.The separation between adjacent carriers is defined by the operatingstandards and is selected to maximize the capacity supported within thelicensed spectrum without excessive interference.

In cellular systems, a plurality of base stations is distributed acrossthe service area. Each base station services wireless communicationswithin a respective cell. Each cell may be further subdivided into aplurality of sectors. In many cellular systems, e.g., GSM cellularsystems, each base station supports forward link communications (fromthe base station to subscriber units) on a first set of carrierfrequencies and reverse link communications (from subscriber units tothe base station) on a second set carrier frequencies. The first set andsecond set of carrier frequencies supported by the base station are asubset of all of the carriers within the licensed frequency spectrum. Inmost, if not all cellular systems, carrier frequencies are reused sothat interference between base stations using the same carrierfrequencies is minimized but so that system capacity is increased.Typically, base stations using the same carrier frequencies aregeographically separated so that minimal interference results.

Both base stations and subscriber units include Radio Frequency (RF)transmitters and RF receivers. These devices service the wireless linksbetween the base stations and subscriber units. Each RF receivertypically includes a low noise amplifier (LNA) that receives an RFsignal from a coupled antenna, a mixer that receives the output of theLNA, a band-pass filter coupled to the output of the mixer, and avariable gain amplifier coupled to the output of the mixer. These RFreceiver components produce an Intermediate Frequency (IF) signal thatcarries modulated data.

In order to improve the signal-to-noise ratio of an RF signal presentedto the mixer by the LNA, the gain of the LNA is adjusted. In adjustingthe gain of the LNA, great care must be taken. While maximizing the gainof the LNA serves to increase the Signal to Noise Ratio (SNR) of the RFsignal, if the LNA gain is too large, the mixer will be driven intonon-linear operation and the IF signal produced by the mixer will bedistorted. Such is the case because a non-linear operating region of themixer resides at an upper boundary of its operating range of the mixer.The input power level at which non-linearity is a problem for the mixeris often referred to as a 1 dB compression level. Thus, it is desirableto have the LNA provide as great an amplification of the received RFsignal as possible prior to presenting the amplified RF signals to themixer without driving the mixer into non-linear operation. During mostoperating conditions, the gain of the LNA may be set by viewing theinput power present at the LNA and by setting the LNA gain to produce anoutput that causes the mixer to operate in a linear region.

However, when intermodulation interference exists, this technique forsetting the LNA gain does not work. Intermodulation interference occurswhen the mixer receives RF carriers (in addition to the desired signal)that cause the mixer to produce intermodulation components at the IF,the same frequency as the desired signal. This problem is well known andis a non-linear phenomenon associated with the operation of the mixer.The intermodulation component at the frequency of the desired signal isa third-order intermodulation component, IM3. In order to minimizeintermodulation interference, the gain of the LNA should be reduced.However, reducing the gain of the LNA also reduces the SNR of the signalproduced by the mixer. Thus, competing operational goals exist by thecompeting goals of increasing SNR by increasing the gain of the LNA andby reducing the effects of intermodulation interference by reducing thegain of the LNA.

While this problem is well known, its solution is not. Some priortechniques have simply avoided high LNA gain when wideband receivedsignal strength (across some portion of the operating range of the RFreceiver) was approximately equal to the narrowband received signalstrength (at the IF). Further, when the wideband received signalstrength was significantly greater than the narrowband received signalstrength, the gain of the LNA was set to a low level. These operationsaddressed the issue of the existence of interferers. However, it did notconsider whether intermodulation interference existed.

Thus, there is a need in the art to improve the operationalcharacteristics of the LNA in order to maximize signal-to-noise ratioand to minimize the effects of intermodulation interference.

SUMMARY OF THE INVENTION

The present invention is directed to apparatus and methods of operationthat are further described in the following Brief Description of theDrawings, the Detailed Description of the Embodiments of the Invention,and the Claims. Other features and advantages of the present inventionwill become apparent from the following detailed description of theembodiments of the invention made with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects and advantages of the presentinvention will be more fully understood when considered with respect tothe following detailed description, appended claims and accompanyingdrawings wherein:

FIG. 1 is a system diagram illustrating a cellular system within whichthe present invention is deployed;

FIG. 2 is a block diagram illustrating a Radio Frequency (RF) unit thatoperates according to the present invention;

FIG. 3A is a system diagram illustrating a portion of the system of FIG.1 in which intermodulation interference is produced;

FIGS. 3B and 3C are diagrams illustrating how the mixer of an RF unit ofa subscriber unit produces an intermodulation interference signal, IM3,when strong adjacent channel interferers are present;

FIG. 4A is a diagram illustrating the structure of TDMA Time Slotsemployed in a system that operates according to the present invention;

FIG. 4B is a diagram illustrating the structure of a Time Slot accordingto the present invention;

FIG. 5 is a logic diagram illustrating generally operation according tothe present invention;

FIGS. 6 and 7 are logic diagrams illustrating in more detail operationaccording to the present invention;

FIG. 8 is a block diagram illustrating the structure of a subscriberunit constructed according to the present invention;

FIG. 9 is a block diagram illustrating the structure of a base stationaccording to the present invention; and

FIG. 10 is a graph showing the power of received signals present at theinput of a mixer during operation according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a system diagram illustrating a cellular system within whichthe present invention is deployed. The cellular system includes aplurality of base stations 102, 104, 106, 108, 110, and 112 that servicewireless communications within respective cells/sectors. The cellularsystem services wireless communications for a plurality of wirelesssubscriber units. These wireless subscriber units include wirelesshandsets 114, 120, 118, and 126, mobile computers 124 and 128, anddesktop computers 116 and 122. When wirelessly communicating, each ofthese subscriber units communicates with one (or more during handoff) ofthe base stations 102 through 112. Each of the subscriber units of FIG.1, both subscriber units and base stations includes radio frequencycircuitry.

The services provided by the cellular system include both voice serviceand data services. Such services are provided according to a cellularnetworking standard such as the GSM standards, the IS-136 standards, theIS-95 standards, the 1xRTT standards, the 1XEV-DO standards, the 1XEV-DVstandards, other operating standards in which communications are carriedon a number of carriers across a frequency spectrum.

FIG. 2 is a block diagram illustrating a Radio Frequency (RF) unit thatoperates according to the present invention and that is present in thesubscriber units 114-128 and/or the base stations 102-112 of FIG. 1. Asshown, the RF unit 200 includes an antenna 202 that couples to atransmit/receive block 204. The transmit/receive block 204 couples totransmit circuitry 206. The transmit circuit 206 receives anIntermediate Frequency (IF) transmit signal, up-converts the IF transmitsignal to an RF transmit signal, and couples the RF transmit signal tothe transmit/receive block 204 that couples the RF transmit signal tothe antenna 202.

The transmit/receive switch 204 also couples the antenna 202 to an RFSAW circuit 208 so that the RF SAW circuit 208 receives an RF receivesignal. The output of the RF SAW circuit 208 is received by low noiseamplifier (LNA) 210. Associated with the LNA 210 is a LNA gain (LNA_G).The LNA 210 amplifies the signal received at its input from the RF SAW208 by the LNA_G receives to produce an output which it applies to mixer212. Mixer 212 mixes a Local Oscillator (LO) input with the output ofthe LNA 210 to produce an IF receive signal. The output of the mixer 212is received by band-pass filter (BPF) 214, which filters the output ofthe mixer 212 within a frequency band of interest. Residing within thisfrequency band of interest is an IF signal that carries modulated data.The output of the BPF 214 is amplified via a Variable Gain Amplifier(VGA) 216. The VGA 216 produces the IF received signal as its output.

According to the present invention, the RF unit 200 includes tworeceived signal strength indicators (RSSIs). A first RSSI, RSSI_A 218,measures the signal strength of a wideband signal produced by the mixer212. The wideband signal whose strength the RSSI_A 218 measures thecombined strength of a plurality of carriers that the RF unit 200operates upon and that are down converted by the mixer. A second RSSI,RSSI_B 220, receives as its input the output of the BPF 214. Thus, theRSSI_B 220 measures the received signal strength within the frequencyband output by the BPF 214, a narrowband signal. The narrowbandfrequency corresponds to the frequency of the IF receive signal thatcontains modulated data intended for a wireless device containing the RFunit 200 (a signal of interest). In a first alternate embodiment, theRSSI_B 220 receives as its input the output of the VGA 216. In a secondalternate embodiment, an RSSI_B 221 is present in a coupled baseband/IFprocessor that is separate from the RF unit 200. Thus, as is illustratedin these alternate embodiments, the RSSI_B need not be directly coupledto the output of the BPF 214 but must be able to measure narrowbandsignal strength in the received signal path.

LNA gain adjustment block 222 receives the measured received signalstrengths from the RSSI_A 218 and the RSSI_B 220. Based upon these twoinputs, the LNA gain adjustment block 222 produces the LNA_G for the LNA210. The LNA gain adjustment block 222 satisfies various competingcriterion. Generally speaking, the LNA gain adjustment block 222 selectsan LNA_G to maximize the Signal to Noise Ratio (SNR) of the IF signalproduced by the RF unit 200 while operating the mixer 212 in its linearrange. Further, the LNA gain adjustment block 222 detectsintermodulation interference and, when detected, adjusts the LNA_G sothat the mixer 212 operates well within its linear range.

The LNA gain adjustment block 222 may be dedicated hardware, may be acombination of hardware and software, or may be implemented in software.Further, the RSSI_B 220, when contained in the RF unit 200, will beimplemented mostly/fully implemented in hardware. However, the RSSI_B221 contained in the baseband/IF processor may be implementedpartially/fully in software.

FIG. 3A is a system diagram illustrating a portion of the system of FIG.1 in which intermodulation interference is produced. The example of FIG.3A shows one possible operating condition of the system of FIG. 1 inwhich a large third order modulation product (IM3) is present. Ofcourse, many various other operating conditions may also produce largeIM3 products. As shown in FIG. 3A, subscriber unit 114 receives forwardlink transmissions from base station 106 at a carrier frequency of 880.2MHz. However, these forward link transmissions from the base station 106to the subscriber unit 114 pass through an obstacle 300 that weakens thereceived signal. Base station 102 transmits forward link signals toother subscriber units at the carrier frequencies of 881.0 MHz and 881.8MHz.

FIGS. 3B and 3C are diagrams illustrating how the mixer 212 of an RFunit 200 of the subscriber unit 114 produces an intermodulationinterference signal, IM3, when the strong adjacent channel interferersare present. FIG. 3B illustrates the mixer input power present withinthe RF unit of subscriber unit 114. As is shown, interfering carrier(INT_A) is present at 881.0 MHz and interfering carrier (INT_B) ispresent at 880.2 MHz. The signal separation between INT_A and INT_B is800 KHz. Further, the separation between INT_A and the desired carrierthat carries the signal of interest at 880.2 MHz is 800 KHz and the LOinput is at a higher frequency than each of the carrier frequencies. Thefrequency separation between the LO and the desired carrier is equal tothe IF.

FIG. 3C illustrates the output power at the mixer of the RF unit in thereceive path for subscriber unit 114. As is shown, interfering signalsINT_A and INT_B produce intermodulation components of the third order,IM3, that coexist with the IF of the desired signal. When the desiredsignal is relatively weak, the IM3 component is relatively large ascompared to the power of the desired signal. The band-pass filter can donothing to remove the IM3 component which is additive to the desiredsignal at the IF. Thus, the RF unit simply passes both the desiredsignal and the IM3 component at the IF to demodulating section of thesubscriber unit 114.

With the relatively large IM3 component at the IF, the demodulatingsection of the subscriber unit 114 will have difficulty extractingmodulated data. Thus, high bit error rates and lost packets may occurwhen the large IM3 component exists. Thus according to the presentinvention, during a guard period when the desired signal is not present,the gain of the LNA 210 is adjusted by the LNA gain adjustment block 222to minimize the impact of the IM3 component by ensuring that the mixer212 operates well within its linear region. These operations aredescribed in detail with reference to FIGS. 4A through 7.

FIG. 4A illustrates the structure of TDMA time slot employed in a systemthat operates according to the present invention. As is shown in FIG.4A, transmissions on any carrier within the wireless communicationsystem occur in time slots. Such is the case in almost all systems butparticularly in TDMA systems. These time slots are transmitted somewhatcontinually by the base station to a subscriber unit during an ongoingcommunication.

FIG. 4B is a diagram illustrating the structure of a time slot accordingto the present invention. As is shown, the time slot 400 includes aguard period 402 and an active transmission period 404, followed by theguard period of a subsequent time slot. During the guard period 402, thecarrier corresponding to the desired signal is not present. During thisguard period, the LNA gain adjustment block 222 operates to adjust theLNA_G according to the present invention.

FIG. 5 is a logic diagram illustrating generally operation according tothe present invention. As shown in FIG. 5, operation commences after theguard period has commenced (step 502). When the guard period commences,a guard period timer may be set to expire when the guard period ends.Then, the wideband received signal strength (RSSI_A) is measured (step504). Then, it is determined whether mixer non-linearity (1 dBcompression) is caused based upon the strength of the wideband signalpresent at the mixer (step 506). If the determination at step 506 isaffirmative, the LNA_G is reduced at step 508 until it is determinedthat the mixer is no longer driven into 1 dB compression due to thewideband signal.

If the mixer is not driven into 1 dB compression due to the widebandsignal, as determined at step 506, it is next determined whether thewideband signal strength, RSSI_A, exceeds a particular threshold(THRS_B) to indicate that the wideband signal alone will cause the mixerto operate in its upper range. If this threshold is exceeded at step510, operation proceeds to step 512 where the narrowband receive signalstrength is measured via RSSI_B (step 512). With both the widebandreceived signal strength and narrowband received signal strength,measured operation is performed to determine whether intermodulationinterference exists (step 514).

In determining whether IM3 exists, IM3 is first characterized byEquation (1) as:

$\begin{matrix}{{{{IM}\; 3\left( {{dB}\; m} \right)} = {{3*{{Pin}({dB})}} - {2*{IIP}\; 3\left( {{dB}\; m} \right)}}}{{where}\text{:}}{{{Pin} = {{Input}\mspace{14mu}{Power}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{Mixer}}};}{and}{{{IIP}\; 3} = {3^{rd}\mspace{14mu}{Order}\mspace{14mu}{Intercept}\mspace{14mu}{Point}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{{Mixer}.}}}} & {{Equation}\mspace{14mu}(1)}\end{matrix}$Thus, when an IM3 component exists, its existence will be revealed by:

-   -   (1) measuring RSSI_B;    -   (2) altering the LNA_G;    -   (3) measuring again RSSI_B; and    -   (4) determining that the change in RSSI_B (dB) was greater than        the alteration in LNA_G (dB). Such is the case because the IM3        component is not a linear function of LNA_G.

If intermodulation interference does exist, the LNA_G is set to optimizethe signal-to-interference ratio of the mixer (step 516) by causing themixer to operate well within its linear range. In one particularembodiment of step 516, the LNA_G is reduced until the IM3 component, asindicated by RSSI_B is lower than a predefined threshold. From both step516 and when RSSI_A did not exceed THRS_B, operation proceeds to step518 wherein the LNA_G is set so that the narrowband signal exceeds aminimum.

FIGS. 6 and 7 are logic diagrams illustrating in more detail operationaccording to the present invention as was described in FIG. 5. In thedescription of FIG. 6, THRS_A, is chosen as −23 dBm, THRS_B is selectedas −43 dBm, THRS_C is selected as −102 dBm and a Delta, D, is selectedat 6 dB. From step 504 of FIG. 5, operation proceeds to step 602 of FIG.6 in the more detailed description of FIG. 6. In such case, the widebandreceived signal strength, RSSI_A, is compared to THRS_A (−23 dBm). IfRSSI_A exceeds THRS_A, operation proceeds to step 604 where LNA_G isreduced by one step. In the particular example of FIG. 6, the LNA_G maybe set at any of 20, 16, 12, 8, 4, and 0 dB. Thus, the gain step is 4dB, the max gain is 20 dB and the minimum gain is 0 dB. From step 604,operation proceeds to step 606 where it is determined whether thecurrent LNA_G is greater than LNA_G_MIN (0 dB). If so, operationproceeds again to step 602. If not, operation ends and the minimum LNA_Gof 0 dB is used until the operation of FIG. 6 is again employed toadjust LNA_G.

When RSSI_A does not exceed THRS_A, operation proceeds to step 608 whereit is determined whether RSSI_A exceeds THRS_B (−43 dBm). If not,operation proceeds to step 702 of FIG. 7. If so, the narrowband receivedsignal strength RSSI_B is measured (step 610). Next, it is determinedwhether RSSI_B exceeds THRS_C (−102 dBm) (step 612). If not, operationproceeds to step 702 of FIG. 7. If so, it is determined whether theLNA_G is greater than the LNA_G_MIN (step 614). If not, operation ends.

If so, the LNA_G is reduced by one step and the narrowband receivedsignal strength is again read (step 616). Next, it is determined whetheran intermodulation interference component IM3 is present (step 618). Inmaking this determination, it is assumed that intermodulation componentIM3 will be revealed when RSSI_B decreases non-linearly with a decreasein LNA_G. Thus, the dB decrease in RSSI_B caused by the dB reduction inLNA_G (measurement at step 610 versus measurement at step 616) iscompared to the dB reduction in LNA_G. If the dB decrease in RSSI_B isnot linearly related to the dB decrease in LNA_G, IM3 is present. In oneparticular embodiment, the inequality of Equation (2) is employed todetect IM3:

$\begin{matrix}{{{{{{{{{RSSI\_ B} - {RSSI\_ B}}’} > \left( {{LNA\_ G} + 3} \right)}{{where}\text{:}}{{{RSSI\_ B} = {{measurement}\mspace{14mu}{at}\mspace{14mu}{step}\mspace{14mu} 610}};}{RSSI\_ B}}’} = {{measurement}\mspace{14mu}{at}\mspace{14mu}{step}\mspace{14mu} 616}};}{and}{{LNA\_ G} = {{LNA\_ G}\mspace{14mu}{at}\mspace{14mu}{step}\mspace{14mu} 610.}}} & {{Equation}\mspace{14mu}(2)}\end{matrix}$

When this equation is satisfied, a third order intermodulation productIM3 is present. If IM3 product is not present, as determined at step618, operation ends. If IM3 is present, as determined at step 618, it isnext determined whether the LNA_G is greater than the minimum LNA_G(step 620). If not, operation ends. If so, the LNA_G is reduced by onestep (step 622) and operation proceeds again to step 610. Therefore,when the mixer is operating in its upper region and IM3 is detected,LNA_G is reduced until RSSI_B does not exceed THRS_C (−102 dBm). Suchadjustment of LNA_G will cause the mixer to operate well within itslinear range.

Referring now to FIG. 7, at step 702, RSSI_B is compared to thedifference between THRS_C (−102 dBm) and Delta (6 dBm). If RSSI_B isless than (THRS_C+Delta), it is next determined whether LNA_G is lessthan the LNA_G_MAX (step 704). If so, the LNA_G is increased by onestep, the RSSI_B is again measured (step 706) and operation returns tostep 702. If at step 702 or 704 a negative determination is made,operation ends.

FIG. 8 is a block diagram illustrating the structure of a subscriberunit 802 constructed according to the present invention. The subscriberunit 802 operates with a cellular system, such at that described withreference to FIG. 1 and according to the operations described withreference to FIGS. 2-7. The subscriber unit 802 includes an RF unit 804,a processor 806, and a memory 808. The RF unit 804 couples to an antenna805 that may be located internal or external to the case of thesubscriber unit 802. The processor 806 may be an Application SpecificIntegrated Circuit (ASIC) or another type of processor that is capableof operating the subscriber unit 802 according to the present invention.The memory 808 includes both static and dynamic components, e.g., DRAM,SRAM, ROM, EEPROM, etc. In some embodiments, the memory 808 may bepartially or fully contained upon an ASIC that also includes theprocessor 806. A user interface 810 includes a display, a keyboard, aspeaker, a microphone, and a data interface, and may include other userinterface components. The RF unit 804, the processor 806, the memory808, and the user interface 810 couple via one or more communicationbuses/links. A battery 812 also couples to and powers the RF unit 804,the processor 806, the memory 808, and the user interface 810.

The RF unit 804 includes the components described with reference to FIG.2 and operates according to the present invention to adjust the LNAgain. The structure of the subscriber unit 802 illustrated is only anexample of one subscriber unit structure. Many other varied subscriberunit structures could be operated according to the teachings of thepresent invention.

FIG. 9 is a block diagram illustrating the structure of a base station902 constructed according to the present invention. The base station 902includes a processor 904, dynamic RAM 906, static RAM 908, EPROM 910,and at least one data storage device 912, such as a hard drive, opticaldrive, tape drive, etc. These components (which may be contained on aperipheral processing card or module) intercouple via a local bus 917and couple to a peripheral bus 920 (which may be a back plane) via aninterface 918. Various peripheral cards couple to the peripheral bus920. These peripheral cards include a network infrastructure interfacecard 924, which couples the base station 902 to a wireless networkinfrastructure 950.

Digital processing cards 926, 928 and 930 couple to Radio Frequency (RF)units 932, 934, and 936, respectively. Each of these digital processingcards 926, 928, and 930 performs digital processing for a respectivesector, e.g., sector 1, sector 2, or sector 3, serviced by the basestation 902. The RF units 932, 934, and 936 couple to antennas 942, 944,and 946, respectively, and support wireless communication between thebase station 902 and subscriber units. Further, the RF units 932, 934,and 936 operate according to the present invention. The base station 902may include other cards 940 as well.

FIG. 10 is a graph showing the power of received signals present at theinput of a mixer during operation according to the present invention.Time increases to the right in the graph, commending with a guardperiod, during which no desired signal is present. However, thermalnoise and an intermodulation interference component IM3, are bothpresent. During a first portion of the guard period (period 1002), theLNA_G is set to a maximum gain and the LNA Gain Adjustment Circuitdetermines that IM3 is present. At transition 1004, the LNA_G is reducedby one step and, resultantly, the IM3 component decreases non-linearlyand the thermal noise increases linearly. Thus, during period 1006, theIM3 component has decreased non-linearly while the thermal noise floorhas increased linearly.

At transition 1008, the LNA_G is reduced by another step. Resultantly,IM3 decreases again non-linearly while the thermal noise floor increaseslinearly. After the transition 1008, during period 1010, the magnitudeof IM3, which is decreasing substantially, meets the level of the noisefloor. At transition 1012, the guard period expires and the desiredsignal is present. At this time, the signal of interest will be presentwith a carrier-to-interference ratio (C/I). This is denoted as C/I(with). If the gain of the LNA had not been adjusted, the C/I ratio(without) would have been less and a coupled modulator would have hadmore difficulty in extract data from the desired signal.

The invention disclosed herein is susceptible to various modificationsand alternative forms. Specific embodiments therefore have been shown byway of example in the drawings and detailed description. It should beunderstood, however, that the drawings and detailed description theretoare not intended to limit the invention to the particular formdisclosed, but on the contrary, the invention is to cover allmodifications, equivalents and alternatives falling within the spiritand scope of the present invention as defined by the claims.

1. A method comprising: (a) determining that a signal of interest is notpresent in a received radio frequency signal at an input of a low noiseamplifier; (b) measuring a wideband signal strength indication at anoutput of a mixer, the mixer coupled to receive an output of the lownoise amplifier; (c) comparing the wideband signal strength indicationto a first threshold value; (d) reducing gain of the low noise amplifierwhen the wideband signal strength indication exceeds the first thresholdvalue, until the wideband signal strength indication no longer exceedsthe first threshold value; (e) determining if the wideband signalstrength indication exceeds a second threshold value; (f) measuring anarrowband signal strength indication after band pass filtering theoutput of the mixer, when the wideband signal strength indicationexceeds the second threshold value; (g) comparing the narrowband signalstrength indication to a third threshold value; (h) reducing the gain ofthe low noise amplifier by a predetermined amount when the narrowbandsignal strength indication exceeds the third threshold value; (i)determining if intermodulation interference is present by measuring thenarrowband signal strength indication for a non-linear gain responseafter reducing the gain of the low noise amplifier by the predeterminedamount; (j) determining if the gain of the low noise amplifier exceeds afirst predetermined minimum, if intermodulation interference isdetected; (k) repeating (f) through (j), provided the gain of the lownoise amplifier exceeds the first predetermined minimum, to establish again setting for the low noise amplifier to operate the low noiseamplifier within its linear region of operation in order to optimize asignal-to-interference ratio of the mixer for subsequent reception ofthe signal of interest, in which a combination of wideband signalstrength indication and narrowband signal strength indication based gainadjustment of the low noise amplifier is employed to compensate forintermodulation interference.
 2. The method of claim 1, wherein reducingthe gain of the low noise amplifier in (d) includes reducing the gain inincremental steps until the wideband signal strength indication nolonger exceeds the first threshold value, provided the gain of the lownoise amplifier is maintained above the first predetermined minimum. 3.The method of claim 1, further including comparing the gain of the lownoise amplifier to ensure that the gain is above a second predeterminedminimum prior to reducing the gain of the low noise amplifier by thepredetermined amount in (h).
 4. The method of claim 1, wherein reducingthe gain of the low noise amplifier in (d) includes reducing the gain inincremental steps until the wideband signal strength indication nolonger exceeds the first threshold value, provided the gain of the lownoise amplifier is maintained above the first predetermined minimum; andfurther including comparing the gain of the low noise amplifier toensure that the gain is above a second predetermined minimum prior toreducing the gain of the low noise amplifier by the predetermined amountin (h).
 5. The method of claim 1, further including comparing thenarrowband signal strength indication to a fourth threshold value, ifthe wideband signal strength indication does not exceed the secondthreshold value in (e) or if the narrowband signal strength indicationdoes not exceed the third threshold value in (g); and increasing thegain of the low noise amplifier in incremental steps until thenarrowband signal strength indication equals at least the fourththreshold value.
 6. The method of claim 1, further including comparingthe narrowband signal strength indication to a fourth threshold value,if the wideband signal strength indication does not exceed the secondthreshold value in (e) or if the narrowband signal strength indicationdoes not exceed the third threshold value in (g); and increasing thegain of the low noise amplifier in incremental steps until thenarrowband signal strength indication equals at least the fourththreshold value, provided the gain of the low noise amplifier does notexceed a predetermined maximum.
 7. The method of claim 1, whereinreducing the gain of the low noise amplifier in (d) includes reducingthe gain in incremental steps until the wideband signal strengthindication no longer exceeds the first threshold value, provided thegain of the low noise amplifier is maintained above the firstpredetermined minimum; further including comparing the gain of the lownoise amplifier to ensure that the gain is above a second predeterminedminimum prior to reducing the gain of the low noise amplifier by thepredetermined amount in (h); and further including comparing thenarrowband signal strength indication to a fourth threshold value, ifthe wideband signal strength indication does not exceed the secondthreshold value in (e) or if the narrowband signal strength indicationdoes not exceed the third threshold value in (g); and increasing thegain of the low noise amplifier in incremental steps until thenarrowband signal strength indication equals at least the fourththreshold value, provided the gain of the low noise amplifier does notexceed a predetermined maximum.
 8. An apparatus comprising: a low noiseamplifier coupled to receive a radio frequency signal during a periodwhen a signal of interest is not present; a mixer coupled to receive anoutput of the low noise amplifier; a band pass filter coupled to anoutput of the mixer; a first signal strength indicator coupled to theoutput of the mixer to indicate a wideband signal strength indication; asecond signal strength indicator coupled after the band pass filter toindicate a narrowband signal strength indication; and a gain adjustmentmodule coupled to: (a) compare the wideband signal strength indicationto a first threshold value; (b) reduce gain of the low noise amplifierwhen the wideband signal strength indication exceeds the first thresholdvalue, until the wideband signal strength indication no longer exceedsthe first threshold value; (c) determine if the wideband signal strengthindication exceeds a second threshold value; (d) compare the narrowbandsignal strength indication to a third threshold value, if the widebandsignal strength indication exceeds the second threshold value in (c);(e) reduce the gain of the low noise amplifier by a predetermined amountwhen the narrowband signal strength indication exceeds the thirdthreshold value; (f) determine if intermodulation interference ispresent by assessing the narrowband signal strength indication for anon-linear gain response after reducing the gain of the low noiseamplifier by the predetermined amount; (g) determine if the gain of thelow noise amplifier exceeds a first predetermined minimum, ifintermodulation interference is detected; (h) repeat (d) through (g),provided the gain of the low noise amplifier exceeds the firstpredetermined minimum, to establish a gain setting for the low noiseamplifier to operate the low noise amplifier within its linear region ofoperation in order to optimize a signal-to-interference ratio of themixer for subsequent reception of the signal of interest, in which acombination of wideband signal strength indication and narrowband signalstrength indication based gain adjustment of the low noise amplifier isemployed to compensate for intermodulation interference.
 9. Theapparatus of claim 8, wherein the gain adjustment module reduces thegain of the low noise amplifier in (b) by reducing the gain inincremental steps until the wideband signal strength indication nolonger exceeds the first threshold value, provided the gain of the lownoise amplifier is maintained above the first predetermined minimum. 10.The apparatus of claim 8, wherein the gain adjustment module furthercompares the gain of the low noise amplifier to ensure that the gain isabove a second predetermined minimum prior to reducing the gain of thelow noise amplifier by the predetermined amount in (e).
 11. Theapparatus of claim 8, wherein the gain adjustment module reduces thegain of the low noise amplifier in (b) by reducing the gain inincremental steps until the wideband signal strength indication nolonger exceeds the first threshold value, provided the gain of the lownoise amplifier is maintained above the first predetermined minimum; andwherein the gain adjustment module further compares the gain of the lownoise amplifier to ensure that the gain is above a second predeterminedminimum prior to reducing the gain of the low noise amplifier by thepredetermined amount in (e).
 12. The apparatus of claim 8, wherein thegain adjustment module further compares the narrowband signal strengthindication to a fourth threshold value, if the wideband signal strengthindication does not exceed the second threshold value in (c) or if thenarrowband signal strength indication does not exceed the thirdthreshold value in (d); and the gain adjustment module increases thegain of the low noise amplifier in incremental steps until thenarrowband signal strength indication equals at least the fourththreshold value.
 13. The apparatus of claim 8, wherein the gainadjustment module further compares the narrowband signal strengthindication to a fourth threshold value, if the wideband signal strengthindication does not exceed the second threshold value in (c) or if thenarrowband signal strength indication does not exceed the thirdthreshold value in (d); and the gain adjustment module increases thegain of the low noise amplifier in incremental steps until thenarrowband signal strength indication equals at least the fourththreshold value, provided the gain of the low noise amplifier does notexceed a predetermined maximum.
 14. The apparatus of claim 8, whereinthe gain adjustment module reduces the gain of the low noise amplifierin (b) by reducing the gain in incremental steps until the widebandsignal strength indication no longer exceeds the first threshold value,provided the gain of the low noise amplifier is maintained above thefirst predetermined minimum; and wherein the gain adjustment modulefurther compares the gain of the low noise amplifier to ensure that thegain is above a second predetermined minimum prior to reducing the gainof the low noise amplifier by the predetermined amount in (e); andfurther compares the narrowband signal strength indication to a fourththreshold value, if the wideband signal strength indication does notexceed the second threshold value in (c) or if the narrowband signalstrength indication does not exceed the third threshold value in (d);and the gain adjustment module increases the gain of the low noiseamplifier in incremental steps until the narrowband signal strengthindication equals at least the fourth threshold value, provided the gainof the low noise amplifier does not exceed a predetermined maximum.